In the last forty years, neuromodulation and neurostimulation implantable technologies have been used extensively for a variety of indications. As such, the components of these systems have developed a significant track record and their actions on the body are reasonably well understood. Many of these systems use implantable pulse generators (IPG's) and electrodes to deliver charge to the site of a biological tissue, i.e., a nerve. By using an appropriate low frequency waveform, the system induces action potentials in a targeted nerve (or nerves) that create the desired effect. It should be noted that the ability to create action potentials does not necessarily require direct contact with the nerve.
However, certain applications may require a direct contact with the nerve. One example is selective stimulation or recording from a portion of a nerve bundle. Another example is the blocking of conduction of action potentials using high frequency signals.
High frequency nerve blocks are immediately reversible, which makes them a more attractive clinical solution for conditions that have traditionally required treatments that are not reversible and permanent, such as nerve transections. Unlike other indications that attempt to selectively recruit nerve fascicles, where current is steered to target portions of the nerve, the conduction nerve block requires a saturation of the nerve with a current field. This saturation effect is best achieved with a circumferential set of electrode bands in a tri-polar configuration surrounding the entire nerve or other multi-pole configurations with the outermost bands at the same potential.
Kilgore and Bhadra have investigated the use of a low voltage, high frequency signal to create a block [Kilgore et al., 2004]. Their research has to date shown that a 5 kHz to 30 kHz balanced biphasic waveform produced a complete motor block in 34 of 34 nerves tested in nerves of various small and large mammals, including dogs. The block was completely reversible in all cases.
Similar results have been achieved in mammals for acute applications but with more variability in results. It has been demonstrated that a major factor in the efficacy and repeatability of the block is the circumferential contact that the electrode has to the targeted nerve. The results described above have been obtained using the spiral cuff electrode, first patented in 1986 by Naples, Mortimer, et al (U.S. Pat. No. 4,602,624). It is a laminated assembly of two Silastic sheets (Dow Corning), with one layer stretched during the glue-up process (Silastic Adhesive). Once the assembly is freed from the press, it naturally curls towards the stretched side. The flat edge is typically long enough so that the cuff makes at least one and half revolutions of the nerve. This seals the cuff to provide an insulation barrier so that current does not leak around the cuff. The two laminates carry platinum electrodes, with windows cut out on the stretched side so that current can be conducted inwards.
Existing spiral cuff electrode do not reliability interface to small nerves. The stiffness of the platinum prevents the electrode from fully conforming to the small diameter of the nerve. The stiffness also does not allow the electrode to be fully adaptive, accommodating post-operative swelling of the nerve, which commonly occurs. Furthermore, the manufacturing process described in the Naples et al. Patent to produce the electrode is hand-labor intensive with low repeatability of key process parameters.